I installed Mesa earlier today, and in the process had to change from XFree86 to X.org (Debian made me). I did what I wanted to do in Mesa, so I uninstalled it and reinstalled Nvidia's drivers using their installer. It seemed to work, but now glgears is only getting about 10 FPS, and other OpenGL apps are just as slow. When I run glxinfo it says that I'm using the Nvidia driver, and all the opengl libraries in /usr/lib seem to be pointing to the right Nvidia libraries. What's wrong?

Have you checked if there are any other libGL files in any of the lib/ directories?

Get rid of mesa, you don't need mesa when you can install the nvidia drivers.

Turns out a process was using 80% of the CPU. I killed it and everything worked fine. Wups.